Article pubs.acs.org/Macromolecules
A Boron Subphthalocyanine Polymer: Poly(4-methylstyrene)-copoly(phenoxy boron subphthalocyanine) Jeremy D. Dang,†,∥ Jessica D. Virdo,†,∥ Benoît H. Lessard,† Elijah Bultz,† Andrew S. Paton,† and Timothy P. Bender*,†,‡,§ †
Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario, Canada M5S 3E5 ‡ Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6 § Department of Materials Science and Engineering, University of Toronto, 184 College Street, Toronto, Ontario, Canada M5S 3E4 S Supporting Information *
ABSTRACT: Boron subphthalocyanines have been explored as functional materials in a wide range of organic electronic applications including organic light-emitting diodes and organic solar cells. They have only, however, been studied as small molecules; there have been no reports of boron subphthalocyanine polymer(s) in the literature. Herein, the synthesis of the first boron subphthalocyanine-containing polymer, poly(4-methylstyrene)-co-poly(phenoxy boron subphthalocyanine), is reported along with its basic physical characterization/properties. The synthesis of the boron subphthalocyanine polymer was only possible via a postpolymerization functionalization approach, where the boron subphthalocyanine moiety is appended to the side chain of a styrene-based prepolymer. The three-step transformation begins with the nitroxide-mediated copolymerization of 4methylstyrene and 4-acetoxystyrene to form poly(4-methylstyrene)-co-poly(4-acetoxystyrene). The acetoxy groups are subsequently deacetylated to afford hydroxyl groups prior to phenoxylation with bromo boron subphthalocyanine to afford the title polymer(s).
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INTRODUCTION Boron subphthalocyanines (BsubPcs) are macrocyclic compounds made up of three isoindoline groups that chelate a single boron atom (1a/1b, Scheme 1).1 Their highly conjugated aromatic 14-π-electron system coupled with their unique bowl-shaped structure gives rise to their unique electronic and optical properties2 such as a strong absorption in the visible spectrum, photoluminescence, electroluminescence, and n-type conductivity.3 This has led to BsubPcs being explored as functional materials in a wide range of organic electronic applications such as nonlinear optics,4 organic field effect transistors,5,6 organic light-emitting diodes,3,7 and organic solar cells.8−11 Despite the widespread use of both small molecules and polymer materials as functional components in organic electronics, all investigations to date have been limited to the study of BsubPcs as small molecule species; there has yet to be a report on the synthesis and physical characterization of a BsubPc-containing polymer. In contrast, there are a number © XXXX American Chemical Society
of reports on normal phthalocyanine (Pc) containing polymers, which differ in the location of the Pc fragments within the macromolecule: within the main chain,12−16 as a side chain (pendant),17−20 or as the main fragment making up a network polymer.21−26 Many semiconducting polymers of interest in the field of organic electronics contain the functional conjugated component within the main chain of the polymer. Examples include poly(paraphenylene)s, poly(thiophene)s, poly(pyrrole)s, poly(ethylene dioxythiophene)s, poly(fluorene)s, poly(carbazole)s, and various donor−acceptor copolymers.27 In each case, the polymer is produced by the polycondensation of two difunctional monomers or a single bifunctional monomer. We can surmise that the lack of precedent for a BsubPc-containing Received: June 19, 2012 Revised: September 12, 2012
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mm path length. Photoluminescence (PL) spectra were recorded on a PerkinElmer LS55 fluorescence spectrometer using a PerkinElmer quartz cuvette with a 10 mm path length. High-pressure liquid chromatography (HPLC) analysis was carried out on a Waters 2695 separation module with a Waters 2998 photodiode array and a Waters Styragel HR 2 THF 4.6 × 300 mm column. The mobile phase used was HPLC grade acetonitrile (80 vol %) and N,N-dimethylformamide (20 vol %). Gel permeation chromatography (GPC) analysis was conducted on a Waters 2695 separation module with a Waters 2998 photodiode array, a Waters 2414 refractive index detector, and two Waters Styragel 5 μm, HR 4E 7.8 × 300 mm column in series. The mobile phase used was HPLC grade THF, which was run at a flow rate of 1.2 mL/min. Gas chromatography (GC) was conducted on a PerkinElmer AutoSystem with a Rtx-35 (Fused Silica) column with dimensions of 15 m × 0.32 mm × 1.00 μm. Helium was used as the mobile phase, and a flame ionization detector was used for detection. Electrochemical measurements were carried out using a Bioanalytical Systems C3 electrochemical workstation. Spec-grade solvents were purged with argon gas at room temperature prior to their use. Three cycles from +1.6 to −1.6 V at a scan rate of 100 mV/s were measured for each sample. Tetrabutylammonium perchlorate (1 M) was used as the supporting electrolyte. Bromo Boron Subphthalocyanine (Br-BsubPc, 1b) Synthesis. 1b was prepared according to a modified literature procedure.30 To an oven-dried three-neck round-bottom flask equipped with a condenser, an addition funnel, and a gas inlet was added phthalonitrile (42.3 g, 0.330 mol, 3.1 equiv), bromobenzene (136 mL), and toluene (362 mL) under argon. To the addition funnel was charged boron tribromide (10.0 mL, 26.5 g, 0.106 mol, 1 equiv), which was subsequently added to the reaction mixture dropwise. An immediate color change to a dark brown was observed, and the reaction was allowed to stir overnight. The following morning, the stirring was turned off, and the reaction was left undisturbed for 1 h. The reaction was gravity filtered, and the solid was immediately washed with methanol (250 mL) and dried in an oven to give 1b (23.1 g, 46%) as a brown solid. 1H NMR (400 MHz, CDCl3) δ 8.89−8.83 (m, 6H), 8.00−7.94 (m, 6H). 2:1 Poly(4-methylstyrene)-co-poly(4-acetoxystyrene) (9a) Synthesis. A flame-dried three-neck round-bottom flask equipped with a condenser, an internal temperature probe, and a gas inlet was purged with nitrogen. Two pipettes were packed with inhibitor remover replacement packing for removing tert-butylcatechol. 4-AS (11.31 g, 70 mmol) was uninhibited by passing it through one pipette and introduced into the reaction flask. 4-MS (16.49 g, 140 mmol) was uninhibited by passing it through the second pipette and introduced into the reaction flask. To the reaction mixture was added TEMPO (0.024 g, 0.15 mmol), followed by BPO (0.025 g, 0.10 mmol). A small amount of 1,2-dichlorobenzene (1.2 mL) was added as an internal standard to monitor the consumption of the monomer via GC. The reaction was purged with nitrogen for 15 min and heated to 125 °C. The reaction progress was monitored by GC and GPC, and the reaction was stopped when the polymer had reached a desirable molecular weight (Mw ∼ 15 000−18 000 Da). The reaction was precipitated into methanol (1 L) and suction filtered to obtain 9a as a white solid (16.2 g, 58%). 1H NMR (400 MHz, CDCl3) δ 7.05−6.17 (br d, 12H), 2.26 (br s, 9H), 1.78 (br s, 3H), 1.35 (br s, 6H). 4:1 Poly(4-methylstyrene)-co-poly(4-acetoxystyrene) (9b) Synthesis. 9b was obtain as a white solid (53%) via the same procedure used for the synthesis of 9a. 1H NMR (400 MHz, CDCl3) δ 7.05−6.17 (br d, 20H), 2.26 (br s, 15H), 1.80 (br s, 5H), 1.35 (br s, 10H). 2:1 Poly(4-methylstyrene)-co-poly(4-vinylphenol) (10a) Synthesis. To a round-bottom flask equipped with a condenser was added potassium hydroxide (1.12 g, 0.02 mol) and water (150 mL). To the solution was added 9a (2.00 g) and isopropanol (100 mL). The reaction mixture was heated to 75 °C for 12 h and analyzed by FT-IR for the absence of the ester signal at 1768 cm−1. The reaction was allowed to cool to room temperature, and the copolymer was precipitated out of solution by neutralizing the reaction mixture via a dropwise addition of 2 M HCl. The product was isolated by suction
Scheme 1. Attempted Synthesis of BsubPc-Containing Styrenic Monomers via the Prepolymerization Functionalization Approacha
Reagents and conditions: (i) X = Cl, R1 = H, R2 = H, toluene, 80 °C, 24 h; (ii) X = Cl, R1 = OCH3, R2 = CH3, toluene 80 °C, 24 h; (iii) X = Br, R1 = H, R2 = H, toluene, 40 °C, 48 h. a
polymer is attributed to the practical improbability of producing a difunctional or bifunctional monomer of the C3v symmetric BsubPc. It has been stated that polymeric organic electronic materials benefit from cost-effective fabrication techniques including rollto-roll solution coating28 and inkjet printing,29 and it is thus desirable to study a polymeric version of BsubPc for direct comparison with its small molecule analogues. Herein, we describe the synthesis of the first polymeric BsubPc, poly(4methylstyrene)-co-poly(phenoxy boron subphthalocyanine), which has been achieved using a postpolymerization transformation strategy. We emphasize the practical importance of this approach and outline the basic physical properties of the novel BsubPc-containing polymer(s).
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EXPERIMENTAL SECTION
Materials. All ACS grade solvents, HPLC grade solvents, potassium hydroxide, sulfuric acid, and sodium bicarbonate were purchased from Caledon Laboratories (Caledon, Ontario, Canada) and used without further purification unless otherwise stated. 2,2,6,6Tetramethyl-1-piperidinyloxy (TEMPO), 4-acetoxystyrene (4-AS), 4methylstyrene (4-MS), benzoyl peroxide (BPO), and boron tribromide were purchased from Sigma-Aldrich Chemical Co. (Mississauga, Ontario, Canada) and used as received. Phthalonitrile was purchased from TCI America (Portland, OR) and used as received. Bromobenzene was purchased from Alfa Aesar (Ward Hill, MA) and used as received. Deuterated chloroform with 0.05% (v/v) tetramethylsilane (TMS) was purchased from Cambridge Isotope Laboratories (St. Leonard, Quebec, Canada) and used as received. Column chromatography was performed with Silica Gel P60 (mesh size 40−63 μm) obtained from Silicycle. Thin layer chromatography (TLC) was performed on aluminum plates coated with silica (pore size of 60 Å) and fluorescent indicator, obtained from Whatman Ltd., and visualized under UV (254 nm) light. Soxhlet extractions were performed using Whatman single thickness cellulose extraction thimbles (25 × 80 mm). Methods. Nuclear magnetic resonance (NMR) spectra were recorded on a Varian Mercury spectrometer at 23 °C in deuterated chloroform, operating at 400 MHz for 1H NMR. Chemical shifts (δ) are reported in parts per million (ppm) relative to tetramethylsilane (0 ppm) for 1H NMR spectra. Coupling constants (J) are reported in hertz (Hz). Spin multiplicities are designated by the following abbreviations: s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet) and br (broad). Fourier transform infrared spectroscopy (FTIR) were performed on a PerkinElmer Spectrum 100 FT-IR spectrometer using pellets prepared with KBr. Ultraviolet−visible (UV/vis) absorption spectra were acquired on a PerkinElmer Lambda 25 UV/vis spectrometer using a PerkinElmer quartz cuvette with a 10 B
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filtration and washed with water (3 × 50 mL), followed by hexane (3 × 50 mL). The crude product was purified by dissolving it in a minimum volume of THF with the assistance of a sonicator, precipitating it dropwise into ice cold stirring hexane (500 mL) via a cotton-plugged pipette, suction filtering the suspension, and washing the solids with water (3 × 50 mL) and hexane (3 × 50 mL) to obtain 10a (1.58 g, 88%) as a beige solid. 1H NMR (400 MHz, CDCl3) δ: 7.10−6.20 (br d, 12H), 2.26 (br s, 6H), 1.86 (br s, 3H), 1.39 (br s, 6H). 4:1 Poly(4-methylstyrene)-co-poly(4-vinylphenol) (10b) Synthesis. To a round-bottom flask equipped with a condenser was added 9b (2.50 g), toluene (60 mL), ethanol (20 mL), and concentrated sulfuric acid (5 drops). The reaction mixture was heated to 75 °C for 12 h and analyzed by FT-IR for the absence of the ester signal at 1768 cm−1. The reaction was allowed to cool to room temperature where sodium bicarbonate (0.80 g) was added to the mixture and stirred for 4 h to neutralize the sulfuric acid. The reaction mixture was filtered to remove the base and the filtrate was concentrated under reduced pressure. The crude product was purified by dissolving it in a minimum volume of THF with the assistance of a sonicator, precipitating it dropwise into ice cold stirring hexane (600 mL) via a cotton-plugged pipette, suction filtering the suspension, and washing the solids with water (3 × 50 mL) and hexane (3 × 50 mL). The copolymer was reprecipitated a second time in hexane as described above and dried in a vacuum oven at 80 °C overnight to afford 10b (3.20 g, 85%) as a white solid. 1H NMR (400 MHz, CDCl3) δ: 7.12−6.11 (br d, 20H), 2.27 (br s, 12H), 1.85 (br s, 5H), 1.38 (br s, 10H). 2:1 Poly(4-methylstyrene)-co-poly(phenoxy boron subphthalocyanine) (11a) Synthesis. An oven-dried three-neck round-bottom flask equipped with a condenser and a gas inlet was purged with argon for 15 min. To the reaction flask was added 10a (1.58 g) and anhydrous chlorobenzene (180 mL). The reaction was purged for 10 min before adding 1b (2.53 g, 5.33 mmol). The reaction mixture was heated to 120 °C under argon, and the reaction progress was monitored via GPC. When the ratio of BsubPc-containing copolymer and unreacted 1b had reached a steady value, the reaction was stopped and was allowed to cool to room temperature. The reaction mixture was concentrated under reduced pressure to a volume of ∼30 mL, precipitated dropwise into ice cold stirring methanol (1 L) via a cotton-plugged pipette, suction filtered, and washed with hexane (3 × 50 mL) and methanol (3 × 50 mL). The crude product was purified by adding it to a cellulose thimble and extracting with methanol in a Soxhlet extraction apparatus setup until the extract was clear. Three additional reprecipitations into ice cold methanol followed by a single reprecipitation into ice cold hexane was done as described above, and the solid was dried in a vacuum oven at 80 °C overnight to afford 11a (1.85 g, 56%) as a dark purple solid. 1H NMR (400 MHz, CDCl3) δ: 8.74 (br s, 6H), 7.69 (br s, 6H), 7.05−6.12 (br d, 8H), 5.72 (br s, 2H), 5.19 (br s, 2H), 2.21 (br s, 6H), 1.85 (br s, 3H), 1.29 (br s, 6H). 4:1 Poly(4-methylstyrene)-co-poly(phenoxy boron subphthalocyanine) (11b) Synthesis. An oven-dried three-neck round-bottom flask equipped with a condenser and a gas inlet was purged with argon for 15 min. To the reaction flask was added 10b (2.00 g) and anhydrous chlorobenzene (250 mL). The reaction was purged for 10 min before adding 1b (1.90 g, 4.00 mmol). The reaction mixture was heated to 120 °C under argon, and the reaction progress was monitored via GPC. When the ratio of BsubPc-containing copolymer and unreacted 1b had reached a steady value, the reaction was stopped and was allowed to cool to room temperature. The reaction mixture was concentrated under reduced pressure to a volume of ∼30 mL, precipitated dropwise into ice cold stirring methanol (1 L) via a cotton-plugged pipette, suction filtered, and washed with hexane (3 × 50 mL) and methanol (3 × 50 mL). The crude product was purified by adding it to a cellulose thimble and extracting with methanol in a Soxhlet extraction apparatus setup until the extract was clear. Three additional reprecipitations into ice cold methanol followed by a single reprecipitation into ice cold hexane was done as described above, and the solid was dried in a vacuum oven at 80 °C overnight to
afford 11b (1.98 g, 60%) as a bright purple solid. 1H NMR (400 MHz, CDCl3) δ: 8.75 (br s, 6H), 7.71 (br s, 6H), 7.05−6.10 (br d, 16H), 5.73 (br s, 2H), 5.11 (br s, 2H), 2.23 (br s, 12H), 1.81 (br s, 5H), 1.33 (br s, 10H).
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RESULTS AND DISCUSSION
As mentioned above, incorporation of a BsubPc unit into a polymer main-chain by a polycondensation strategy would be synthetically challenging due to the C3v symmetric nature of BsubPc. We therefore aimed to attach the BsubPc as a pendant group onto an inert polymer backbone. The obvious first step was the synthesis of a BsubPc molecule containing a vinyl functional group, which could then be polymerized using either conventional or RDRP31 methods (for example, nitroxidemediated polymerization (NMP)32). This prepolymerization functionalization approach has been successfully employed for the synthesis of a range of monomers bearing functional pendants, including acrylate monomers bearing Disperse Red 133,34 and various other azobenzene pendants,35 a vinylbenzene monomer bearing carbazole,37 and a p-cyanomethylstyrene modified with a number of benzaldehyde pendants.37 In our case, we targeted a styrene-based polymer as styrene is well-known to not interfere with charge transport while other polymers and their associated dipoles might hinder charge transport by the so-called pinning effect (e.g., acrylates).38 It has been shown many times that chloro boron subphthalocyanine (Cl-BsubPc, 1a, Scheme 1) can be axially substituted with a phenol or its derivative in refluxing toluene, chlorobenzene, or other aromatic solvents.41−43 Thus, our first attempt to synthesize a monomer of BsubPc utilized 4-vinylphenol. Despite its structural simplicity, 4-vinylphenol is not commercially available and thus was obtained through the hydrolysis of 4-acetoxystyrene (4-AS) with aqueous potassium hydroxide at 0 °C and either stored in the refrigerator prior to use or used immediately after preparation.44 Reaction of 4vinylphenol with 1a resulted in autopolymerization of the 4vinylphenol at a temperature of 80 °C, which is lower than what would be required to facilitate the production of monomer 2a (Scheme 1). Similar observations were seen for the reaction of 1a with 2-methoxy-4-propenylphenol or isoeugenol in an attempt to make monomer 2b (Scheme 1). From these experiments, it was apparent that lower temperatures were required for the phenoxylation of 1a to overcome the issue of premature autopolymerization. However, using low reaction temperatures will not permit phenoxylation of 1a to proceed. We thus turned to the use of the more reactive Br-BsubPc (1b, Scheme 1). Reaction of 1b with 4vinylphenol in toluene at 40 °C formed several products that included a compound with a different HPLC retention time than 1b and a UV−vis absorption profile characteristic of a BsubPc derivative as well as a polymer (detected by GPC) with a molecular weight of ∼20 000 Da. Attempts to isolate pure compounds from this mixture via silica gel column chromatography were unsuccessful due to the elution of the side products and what seemed to be the result of further reaction promoted by the silica gel. We supposed that the polymer was likely formed by a polymerization catalyzed by HBr acid, a side product liberated from the phenoxylation of 1b. In a second attempt, the experiment was repeated in the presence of pyridine so as to neutralize any HBr formed during the reaction. Premature polymerization was indeed inhibited and a compound with the same HPLC retention time and UV−vis absorption profile as the previous base-free experiment was C
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Scheme 2. Synthesis of BsubPc-Containing Copolymers via the Postpolymerization Functionalization Approacha
a
Reagents and conditions: (i) aqueous ammonium hydroxide, isopropanol, reflux, 18 h; (ii) 1a (X = Cl)/1b (X = Br), 1:1 (v/v) N,Ndimethylacetamide:1,2-dichlorobenzene, reflux, 24 h; (iii) method 1 (6a): aqueous ammonium hydroxide, isopropanol, reflux, 70 h; method 2 (9a): aqueous potassium hydroxide, isopropanol, reflux, 12 h; method 3 (9a and 9b): sulfuric acid, toluene/ethanol, reflux, 12 h; (iv) method 1 (7a): 1b, toluene/chlorobenzene, reflux, 2 h; method 2 (10a and 10b): 1b, chlorobenzene, 120 °C, ∼24−48 h.
synthesized poly(4-vinylphenol) in house using NMP. Specifically, we produced the homopolymer by homopolymerization of 4-AS using benzoyl peroxide (BPO) as the initiator and 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) as the mediator at a BPO:TEMPO ratio of 1:1.5. Intermediate poly(4-AS) (3, Scheme 2) had the following characteristics: Mw = 21 600; PDI = 1.32. Subsequent hydrolysis of 3 to poly(4-vinylphenol) (4, Scheme 2) was accomplished using aqueous ammonium hydroxide in an approach analogous to that of Barclay et al.45 Subsequent reaction of 4 with 1a or 1b turned out to be problematic since polymer 4 was not soluble in aromatic solvents such as toluene or chlorobenzene, solvents commonly used for phenoxylation of halo-BsubPcs.22 Instead, a 1:1 (v/v) mixture of 1,2-dichlorobenzene and a polar aprotic solvent such as DMF or DMAc was needed to solubilize polymer 4. Addition of either 1a or 1b with heating under a positive pressure of argon caused some initial conversion/ partial substitution to the desired BsubPc-containing polymer 5 (as could be seen in the GPC chromatogram that the polymer obtained the characteristic UV−vis absorption profile of a BsubPc derivative), but significant decomposition of the BsubPc chromophore occurred well before 50% substitution. The unidentified decomposition product(s) were green in color, and it was suspected that the decomposition was a result of the known disproportionation and ring expansion of BsubPc
formed. However, the compound could not be isolated even after successive purification via silica gel chromatography for what we assume are the same reasons outlined above. Before additional effort was put into obtaining a BsubPc vinyl monomer we first wanted to determine whether or not the BsubPc group is a radical scavenger. Therefore, two 4methylstyrene (4-MS) homopolymerizations were carried out under nitroxide-mediated polymerization (NMP) conditions, where one of the homopolymerizations was performed with the addition of 0.04 wt % 1b (as an additive, not as a monomer). Even with a minimal amount of 1b in the homopolymerization mixture a significant termination was observed, as evident by the loss of linearity in the scaled conversion (ln((1 − X)−1)) versus time plot (see Supporting Information, Figure S1). This termination therefore indicates that 1b and perhaps more generally BsubPcs are radical scavengers. Because of the challenges associated with obtaining a styrenic monomer of BsubPc and its apparent ability to scavenge radicals, we turned to a postpolymerization functionalization approach (Scheme 2) whereby 1b would be reacted with a preformed polymer (prepolymer) to produce the final BsubPccontaining polymer/macromolecule. Using this approach we first tried to functionalize the prepolymer poly(4-vinylphenol) with BsubPc. Rather than using commercially available poly(4vinylphenol), which has a very large polydispersity (∼6), we D
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overall reaction time was quite long requiring ∼3 days for complete hydrolysis. Next the reaction of 7a with 1b was explored. The desired reaction solvent was either toluene or chlorobenzene. Using either would only form a solution at 2−4 wt % of polymer 7a even at high temperatures. This was significantly more dilute than the typical phenoxylation reaction conditions used. Given the SN1 nature of the reaction, the low concentration would severely limit the reaction rate, and thus the reaction of 7a with 1b was only attempted due to its known higher reaction rate.27 The reaction was monitored by GPC and after 90 min the reaction had stalled as indicated by the presence of unreacted and unchanging quantities of 1b. Removal of the mother liquor and isolation of the solubilized polymer showed that only ∼5% of the polymer had stayed in solution. GPC analysis of the isolated polymer revealed that it did contain BsubPc (based on the characteristic UV−vis absorption spectrum) but had a Mw of 20 000 Da. Thus, based on this observation, fractionation had occurred, and given the precipitation, a target Mw of 40 000 was too high when limited to the use of toluene or chlorobenzene as solvents. The synthesis was repeated with a target Mw of 15 000−18 000 in an attempt to ensure complete dissolution during the postpolymerization reaction process in either toluene or chlorobenzene. Again, using NMP, 4-MS and 4-AS were copolymerized in a 2:1 (mol/mol) ratio this time with a targeted Mw of 15 000−18 000. The evolution of molecular weight and polydispersity was observed over time (Figure S2). Monomer consumption was also measured over time using gas chromatography (GC) against an internal standard of 1,2-dichlorobenzene (Figure S3). In this case, 4-AS was consumed at a slightly higher but comparable rate to 4-MS, indicating approximate equivalence in their reactivity and the production of a random copolymer. Noting that copolymer 9a still had somewhat low solubility in toluene and chlorobenzene, we also synthesized a 4-MS/4-AS copolymer in a 4:1 (mol/mol) ratio to produce 9b. In the synthesis of both 9a and 9b, the process produced copolymers with Mw that varied from batch-to-batch and, in some cases, the target Mw was overshot especially at slightly larger scale. Despite this, copolymer 9a (Mw = 30 800, PDI = 1.85) and 9b (Mw = 25 200, PDI = 1.75) were both carried forward to the subsequent deacetylation reaction. Deacetylation was carried out via base hydrolysis of 9a to give 2:1 poly(4-methylstyrene)-co-poly(4-vinylphenol) (10a, Mw = 27 600, PDI = 2.29) and via acid-catalyzed transesterification of 9b to give 4:1 poly(4-methylstyrene)-copoly(4-vinylphenol) (10b, Mw = 24 200, PDI = 1.88). Different reaction conditions were employed for the deacetylation reaction due to the poor solubility of 9b in aqueous basic solution. Copolymer 9a can also be converted to 10a using the same acid-catalyzed transesterification reaction used to deacetylate 9b. The deacetylated copolymers 10a and 10b were subsequently reacted with 1.2 equiv of 1b in chlorobenzene at 120 °C, and the reaction progress was monitored by GPC (Figures S4 and S5) until the ratio of BsubPc-containing copolymer (RT ∼ 13.0−13.2 min) and unreacted 1b (RT ∼ 16.7−17.0 min) had reached a steady value. Although the GPC retention time for the polymer peak did not shift with reaction progress, phenoxylation was known to occur based on a change in the UV−vis absorption profile for the polymer peak from that of a standard styrenic polymer to that which is characteristic of a BsubPc compound (Figure 1). Each of the 2:1 (11a, final Mw = 8700, PDI = 5.65) and 4:1
derivatives,46 although we could not isolate/purify the decomposition products for detailed characterization. We can, however, surmise that the decomposition is a result of the presence of the polar aprotic solvent or the combination of the aprotic solvent and the liberated HCl or HBr, as 1a is stable under refluxing dichlorobenzene for periods of time exceeding 100 h47 and 1b is stable in refluxing toluene for periods of at least 5 h.48 We also verified that decomposition of the BsubPc occurred regardless of the source or batch of prepolymer used including if the prepolymer is simply commercially available poly(4-vinylphenol). We can thus eliminate the presence of nitroxide end-groups as another possible source of the decomposition. As a final confirmation of the effect of polar aprotic solvents, 1b was reacted with 5 equiv of phenol under standard conditions except using 1:1 (v/v) of toluene:DMAc. In this case decomposition of the BsubPc was also observed, definitely proving that it is the presence of the polar aprotic solvent possibly combined with the presence of the liberated HBr that causes the decomposition of the BsubPc chromophore. Thereafter, we shifted our investigation to the use of copolymers as prepolymers rather than poly(4-vinylphenol) homopolymers. The aim was the inclusion of a selected comonomer that would produce a 4-AS-containing prepolymer, which upon hydrolysis would produce a phenol-containing prepolymer that was soluble in solvents such as toluene and/or chlorobenzenesolvents we knew would not promote the decomposition of the BsubPc chromaphore. As measured reactivity ratios were not widely available for 4-AS, we used the “patterns of reactivity” method to examine a variety of potential comonomers (Table S1). Parameter values (ri,s, πi, ui, and vi) for most monomers needed for this method were obtained from the Polymer Handbook49 and used in the patterns of reactivity calculations directly (eqs S1−S3). In considering candidate monomers for copolymerization with 4-AS, several selection criteria were used. (1) The monomer, as stated above, should produce a copolymer with 4-vinylphenol that is soluble in solvents such as toluene, chlorobenzene, or dichlorobenzene. (2) The monomer should produce a polymer that is insoluble in methanol and/or hexanes to facilitate work-up via precipitation and Soxhlet extraction. (3) The monomer should not be an ester since hydrolysis is necessary to transform the acetoxy groups of 4-AS to the hydroxyl group of 4-vinylphenol and such conditions would obviously also hydrolyze any other ester present. Thus, on consideration, only one monomer fit these criteria and was selected for copolymerization with 4-AS:4-MS. The predicted reactivity ratios for 4-AS and 4-MS are closest to unity, and therefore this monomer pair is likely to produce a copolymer with a random tendency. Thus, we initially set an arbitrary target of Mw = 40 000 for a 4-AS/4-MS copolymer. The arbitrary target was thought to be high enough to ensure good film-forming properties of the resulting polymer. Using NMP, 4-MS and 4-AS were copolymerized at a charged ratio of 2:1 (mol/mol, Scheme 2). This successfully yielded copolymer 6a with Mw = 41 000 and PDI = 1.44. GC analysis confirmed an equal rate of consumption for 4-AS and 4-MS, thus verifying that 6a is a random copolymer. Hydrolysis of 6a to 7a was accomplished by suspending 6a in isopropanol and heating to a reflux with excess ammonia hydroxide. As hydrolysis proceeded, the copolymer became increasingly soluble eventually forming a clear and homogeneous solution. The E
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Figure 2. Ultraviolet/visible absorption (blue) and photoluminescence (PL, red) spectra of 3,4-dimethylphenoxy boron subphthalocyanine and polymers 11a and 11b (as indicated).
chromaphores are partially associated in solution. Photoluminescence (PL)/fluorescence emission spectra were also acquired for the polymers 11a and 11b (Figure 2) in nondeaerated toluene using a λexcitation of 564 nm. The PL spectra show that 11a and 11b have λmax of emission at 573 and 575 nm, respectively, which is again in the range typically observed for small molecule phenoxy-BsubPcs. The fluorescence quantum yields (φ) for 11a and 11b were found to be relatively low at 0.12 and 0.06, respectively, relative to a standard of phenoxy-dodecafluoro-BsubPc (F12BsubPc; eq S4). This is significantly lower than the ∼0.50 that is commonly observed for phenoxy-BsubPc derivatives.3 While the Stokes shift is small, the low quantum yield clearly indicates that other processes are present within the polymer, except the degeneration of the singlet to the ground state perhaps resulting from the aforementioned partial association of the BsubPc chromaphores in solution. In addition, fluorescence excitation spectra were obtained for polymers 11a and 11b and the model compound 3,4dimethylphenoxy-BsubPc in nondeaerated toluene using an observational λemission 573 and 575 nm, respectively (Figure S7). The excitation spectrum for 3,4-dimethylphenoxy-BsubPc is practically identical to that of the absorption spectrum (Figure S7a). The excitation spectrum for polymers 11a and 11b closely matches the respective absorption spectrum; however, each are slightly red-shifted relative to the absorption spectrum (Figure S7b,c), indicating only a fraction of the absorbing chromaphores are responsible for the observed photoluminescence. These observations coupled with the broadened absorption spectrum and low quantum yield of photoluminescence suggest further study of the photophysics of the BsubPc-containing polymers is warranted. Cyclic voltammetry (CV) of 11a and 11b was carried out to determine the energy levels of the frontier orbitals (the HOMO and LUMO). Initial experiments using Ag/AgCl as the reference electrode, Pt disk as the working electrode, Pt wire as the counter electrode, and tetrabutylammonium perchlorate in either DCM or THF as the electrolyte did not show any reduction or oxidation process present for either 11a or 11b. It was suspected that the concentration of these copolymers in DCM or THF was too low to be detected due to their generally poor solubilities. To address this issue, the Pt disk was coated
Figure 1. GPC chromatograms and extracted UV−vis absorption spectra (inset) of boron subphthalocyanine polymers 11a (a) and 11b (b).
(11b, final Mw = 17 600, PDI = 8.34) poly(4-methylstyrene)-copoly(phenoxy boron subphthalocyanine) were isolated and purified via precipitation and a Soxhlet extraction. The GPC spectrum of 11a and 11b (Figure 1) both showed a tailing effect, signifying a potential solubility issue with these copolymers. As a consequence of this, high PDIs were obtained, which were assumed to be inaccurate based on the fact that the PDIs for 9a and 9b were significantly lower (
